1. Field of the Invention
The invention relates to a sputtering cathode with a rotary target.
2. Description of the Related Art
Cathodic sputtering is a well known process for the application of thin, uniform films of a coating material to various substrates.
Cathodic sputtering, which is more particularly described in U.S. Pat. No. 2,146,025, is carried out by means of a cathode connected to an electric power supply and placed in a vacuum enclosure, into which are injected traces of gases (e.g. argon). A target made from the material to be sputtered is mounted on the cathode and is bombarded with high-energy ions produced by the meeting of the gas molecules dispersed in the enclosure and electrons accelerated by the electrical field and magnetically confined close to the target.
The gaseous ions with a high kinetic energy are precipitated on the target with an adequate energy to remove particles therefrom. These particles are condensed on the substrate to be coated, which is positioned in their path and thus a fine coating of material is deposited thereon. This coating can consist of a metal or an alloy and reference is then made to a metal sputtering procedure.
It is also possible to deposit a substance formed by chemical reaction between the target material and a reactive gas introduced in small quantities into the sputtering chamber and reference is then made to reactive mode sputtering.
The rotary cathode according to the invention can advantageously be used with both the metal and reactive sputtering methods, but it is particularly suitable for use in the reactive mode.
The efficiency of vacuum sputtering or atomization equipment was initially very poor, due to the limited probability of ions and electrons meeting in the enclosure. By confining the electrons in the vicinity of the target, there is a significant increase in the ionization level and it is possible to work at a lower pressure, thus increasing the purity of the deposits.
U.S. Pat. No. 4,166,018 describes a planar target cathode behind which are arranged magnets in such a way as to form a confinement zone which is closed on itself in the manner of a oval track and in which the electrons are trapped. This leads to a significant increase in the collision rate with the gaseous atoms, i.e. impacts of ions on the cathode, so that the sputtering rate rises.
In such a cathode, the target undergoes local groove-like erosion, which follows the magnetic track. This makes it necessary to frequently replace the target and the usage rate of the target material is low.
Different processes have been used in order to better distribute the erosion over the extent of the target. Thus, in U.S. Pat. No. 4,525,264, a variable magnetic field is superimposed on that of the magnetic confinement circuit positioned behind the target.
U.S. Pat. No. 4,422,916 describes a cathode having a target mounted on a structure in the form of a rotary cylinder, magnetic confinement means being arranged along a generatrix within the cylindrical structure. Wear to the target is rendered uniform by the angular displacement of the erosion zone. As the confinement means are arranged within the cylindrical structure, the latter must necessarily have large dimensions. As said structure is also filled with a cooling fluid, due to its own weight it is subject to high mechanical stresses. Therefore the usage rate of the target cannot generally exceed 40%.
Moreover, the magnets and the parts of the magnetic circuit, confined in the internal volume of the cylindrical structure, form an obstacle to the circulation of the cooling fluid, mainly in the area where target heating takes place. Therefore the cooling is not very satisfactory, specifically at the point where it must be very efficient. In addition, as the magnets and the magnetic pieces are constantly immersed in this cooling fluid, which is normally water, necessarily corrosion problems occur.